FIGS. 1A and 1B are schematic cross-sectional views illustrating partial steps of a method of fabricating a backside-illuminated image sensor according to prior art. Firstly, as shown in FIG. 1A, a first substrate 102 is provided. The first substrate 102 has a first side 104, which is defined as a front side where a sensing unit is to be disposed, and a second side 106, which is defined as a backside where an optical unit is to be disposed. Accordingly, a sensing structure 120 including a photodiode layer 108 and a metal wiring layer 110 is formed on the first side 104 of the first substrate 102. Subsequently, a second substrate 112 is bonded to the first substrate 102 at the first side 104 with the sensing structure 120.
As shown in FIG. 1B, for allowing the photodiode layer 108 of the sensing structure 120 to receive external light from the backside (i.e. the second side 106), the first substrate 102 is thinned from the second side 106. Then a color filter layer 116 and a microlens layer 118 are sequentially formed on the thinned first substrate 102 at the second side 106. While forming the color filter layer 116 and the microlens layer 118, a highly reflective pattern is formed in the wiring layer, serving as an alignment mark to align with a photo mask used in the process. If alignment is well done, positioning light L penetrates through the color filter layer 116 and the first substrate 102 will be reflected by the alignment mark 114 formed in the wiring layer 110 to the surroundings through the first substrate 102 and the color filter layer 116. Since the intensity of the positioning light L could be weakened to a level insufficient to make effective alignment after penetrating the color filter layer 116 and the first substrate 102 twice, it would be difficult to assure of the alignment of the color filter layer 116 and the microlens layer 118.